AJP - Cell Physiology, Vol 255, Issue 3 C278-C290, Copyright © 1988 by American Physiological Society
Na+-Ca2+ exchange in regulation of contractility in canine cardiac Purkinje fibers
J. K. Sonn and C. O. Lee
Department of Physiology and Biophysics, Cornell University Medical College, New York, NY 10021.
To study Na+-Ca2+ exchange, intracellular Na+ activity (aiNa), twitch
tension, and transmembrane potential were simultaneously measured in canine
cardiac Purkinje fibers driven at a constant rate (1 Hz) in the absence and
presence of strophanthidin (5 X 10(-7) M) at normal, low, and high
extracellular [Na+] ([Na+]o) or [Ca2+] ([Ca2+]o). Intracellular Ca2+
activity (aiCa) of the fibers was also measured in a normal Tyrode
solution. Reductions of [Na+]o by 20, 40, and 60% decreased the ratio of
extracellular Na+ activity (aoNa) and aiNa in the steady state but steeply
increased twitch tension. This finding is consistent with the view that a
decrease in aoNa/aiNa increases intracellular Ca2+ through Na+-Ca2+
exchange. In further agreement with this view, a Na+-free solution
virtually depleted intracellular Na+ and increased the resting tension of
the fibers. The slope of the relation of the logs of twitch tension and
aiNa that was determined at normal [Na+]o and [Ca2+]o may reflect the
properties of the Na+-Ca2+ exchange. Slope of log tension-aiNa relationship
decreased when reducing [Na+]o or increasing [Ca2+]o had decreased the
level of aiNa. On the other hand, the slope increased when a rise in [Na+]o
or a reduction in [Ca2+]o had increased the level of aiNa. These results
indicate that as the aiNa level increased, slope of tension-aiNa relation
increased, which suggests that Na+-Ca2+ exchange may depend on level of
aiNa.(ABSTRACT TRUNCATED AT 250 WORDS)
